Sacred oil is a vial of blessed olive oil that is used to make pyre logs. It is obtained from the Shades of Mort'ton minigame, by using vials of olive oil on the fire altar, once the temple has been fully repaired and the altar lit. Converting olive oil to sacred oil costs sanctity points, which are gained by repairing the temple's walls or by killing the loar shades that attack the temple. Each dose of olive oil costs about 0.9% sanctity to convert; a 4-dose vial of olive oil will cost about 3.6% sanctity to convert into sacred oil. Additionally, players must have at least 10% sanctity in order to convert olive oil into sacred oil.

Sacred oil's sole normal use is to create pyre logs, which may be used to cremate the corpses of vyres or the remains of various shades. Pyre logs are made by adding sacred oil to logs, which grants between 12 and 16 Firemaking experience per set of logs, depending on the tier of logs used. Additionally, different logs require different amounts of sacred oil, with low-end logs requiring 2 doses and high-end logs requiring 4 doses. One dose of sacred oil is also used during The Curse of Arrav.

Cremating vyre corpses in the Columbarium (after Legacy of Seergaze) allows players to improve the effectiveness of the Ivandis flail and blisterwood weapons. The spirits of the vyres will also reward players with keys that may be redeemed for reward items in the Columbarium. Cremating shade remains in Mort'ton is necessary to obtain various keys that allow access to the Shade Catacombs and the reward chests within. In both cases, Firemaking and Prayer experience are gained in the process.

Producing sacred oil can be a money-making option. 3-dose vials of olive oil may be purchased in large quantities from Razmire Keelgan's general store in Mort'ton, once he is cured of the affliction, for 26 coins each. A 3-dose vial of sacred oil is worth 7,168 coins on the Grand Exchange, a profit of 7,142 coins per vial. It is not recommended to buy olive oil on the Grand Exchange, as it is much more expensive at 561 coins for a 3-dose vial.

This money-making method is available to any player who can complete the Shades of Mort'ton quest, though players who have unlocked the bank in Burgh de Rott during In Aid of the Myreque will have an easier time stockpiling olive oil. Players who own Drakan's medallion, obtained during The Branches of Darkmeyer, will have an even easier time. It is possible to gain the necessary sanctity by killing the loar shades and/or repairing the temple, which will give combat and Crafting experience respectively. However, the shades are aggressive to players with a combat level of 80 or lower. For such players, combat gear is recommended regardless of how the sanctity is earned.